Marine-based seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process.
The acquisition of data in marine-based seismic methods usually produces different results in source strength and signature based on differences in acquisition configuration and sub-surface conditions. Further data processing and interpretation of seismic data can be improved when the data collection methods produce results with the greatest degree of repeatability. For example, determining during an acquisition survey that a problem has developed that will degrade the results of the acquisition survey by generating imaging with a low degree of repeatability is important to producing the highest quality seismic imaging.
Fold maps indicate the ability of a seismic survey to illuminate the sub-surface. In essence, these maps ought to describe where seismic reflections occur in depth and how redundant they are. Conventional fold maps are counted on common-midpoints and in-fill decisions made solely upon surface geometrical criterion.
However in laterally heterogeneous media or for dipping reflectors, the midpoint does not stand anymore for the reflection point. Thus, whenever knowledge about sub-surface velocity model becomes available (e.g., from geological a priori or processing of vintage surveys), true mapping should be achieved which takes into account wave-paths distortion during propagation through the sub-surface. The hit-count of reflection paths must be restored in common-reflection points at given depth horizons to access true illumination on targets.
Recently, so-called 4D or time-lapse surveys have become an important addition to the product offerings of seismic survey companies. In 4D surveys, a first survey taken at a first time operates as a baseline to indicate the potential presence/absence of hydrocarbon deposits in a given area. A second survey, taken later, operates to indicate the potential presence/absence of hydrocarbon deposits in the same geographical area, e.g., after removal of the hydrocarbons has occurred. By comparing the two surveys, a 4D picture (where time is the fourth dimension) can be developed which can be used for a number of purposes, e.g., to determine the continued viability of a hydrocarbon field, where to drill, etc. However, in order for a 4D survey to be accurate, the first and second surveys need to be performed in a very similar manner, e.g., shot position, receiver position, etc. This gives rise to a need to make surveys highly repeatable and to determine when subsequent surveys are not accurate repetitions of an earlier, baseline survey.
Thus, mis-positioning between surveys (due to source deviation or streamer feathering) may induce biased perturbations that need to be assessed. Beyond geometrical criterion, monitoring of target illumination provides a geophysical criteria with which to evaluate seismic repeatability.
It has been suggested in the literature to perform full fold mapping on depth-horizons using ray theory, either from hit-count or band-limited Fresnel zone. Such maps can be used for quality control of seismic acquisition data, although they may be insufficiently discriminant. Additionally, correspondence between illumination misfits and associated shooting positions is no more obvious, impeding easy localization for re-shoots. Stated differently, attempts to detect and correct for unacceptable repeatability of seismic images have involved the use of illumination maps for seismic coverage analysis but these attempts are unable to disclose the shot position associated with the unacceptable repeatability location, therefore losing the value of reshooting only locations where repeatability issues occurred.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, and provide the ability to quickly determine if shot images are within acceptable repeatability limits and reshoot only those shot positions that fall outside of the previously described limits.